REAR SPOILERS: Length-'Angles'

[brucepick]

[Pendragon - Yes, I've also considered adding an upper short spoiler to the scheme. To get the air following the template early.]

OK - I'm going to attempt to guess at what happens here. Like the kid in class "I know, I know!!" but guessing...

Those cars that have a truncated teardrop template design such as Prius and Insight (both gens, I think) have very low cd as a result of it. If the same or better cd could have been gotten with a notchback, even a modified notchback of some sort, I'm sure the designers would have gotne that route, and would have had a much easier time selling cars with a more conventional design.

So my guess is - whatever the reason, a body that follows the template actually does give lower cd than a notchback with carefully tailored curves and angles. Now, if we "fake" the template by adding a spoiler or false trunk lid that meets the template line - well, I'm not sure. But aerohead keeps pointing us to the template and I suspect he's right on that.

My guess is that the flow that comes off the trunk lid and parallel sides exits straight rearward and so does not bring top-bottom-sides air flows back together. But with a truncated teardrop or MAYBE even with a "faked" teardrop like we're considering, the airflow is being coaxed to an angled flow as per the template, so the streams are encouraged to rejoin each other with less turbulence.

Note the Civic coupe trunk is angled slightly downwards to the rear. I think they're trying to get some downward angle on that air, but it's just not very much. I do think that a bit of a down angle on a false trunk lid would be good. Just don't angle down more than the template shows, is my guess. On a typical spoiler, a down angle would catch some air and give lift. But a false lid I think can be angled down a bit, as long as there's no significant air flow passing below it.

[COcyclist]

Originally Posted by brucepick

Those cars that have a truncated teardrop template design such as Prius and Insight have very low cd as a result of it. If the same or better cd could have been gotten with a notchback, even a modified notchback of some sort, I'm sure the designers would have gone that route, and would have had a much easier time selling cars with a more conventional design.

So my guess is - whatever the reason, a body that follows the template actually does give lower Cd than a notchback with carefully tailored curves and angles.

Exactly. From AeroHead...

"*The best 'idealized' spoiler could have significant potential for drag reduction,although due to the turbulence associated with the separated flow above the spoiler,it could never achieve the drag reduction of the K-form 'Template' roofline, which is devoid of such turbulence.

*Turbulence cannot be converted to useful pressure regain,only heat,during viscous attrition as the eddies are eventually worn down by calmer surrounding air. It's a second law of thermodynamics thing we strive to avoid."

In a perfect low drag/high mpg world, we would all drive cars that taper smoothly to the rear like a 1/2 ice cream cone or Dave Cloud's Dolphin "Ultimate Aerodynamic Car" but compromises are made for length, styling, parking and visibility. So we end up with counter-rotating vortexes adding drag to our cars. I am excited to see California98Civic testing some ideas and seeing some improvement. It is a step in the right direction but still a compromise.

This would be better IMO but not too practical for most people.

http://ecomodder.com/forum/showthrea...-ii-13037.html

[aerohead]

Originally Posted by California98Civic

This raises questions for me. The tuft testing I have done of my sixth gen Civic coupe shows a recirculation zone for about 4 or 5% of the lid/window area near the spine of the car. Air reattaches a few inches before detaching from the trailing edge of the trunk lid. Good. And better than I thought. Certainly a solid well-placed surface is better than a recirculation zone for allowing air to slip along, no? Certainly the longer, descending-angled raised lid and lip would slow the air more than the 4" where the air currently reattaches, therefore gaining the pressure benefits you describe here, no? It seems to me the key is making sure the contours of a raised lid do not add drag, and if they don't that a 2% drag reduction should be possible. Modest, but measurable in FE terms in a careful test, no?

California,a sketch would really help.I think I understand where you're going.
Circulation is bad.Energy of rotation cannot be converted to pressure.
Even if the air strikes the trunklid,while reducing lift,it may still impart sizeable drag due to where the air separated.
The spoiler,if its trailing edge is near the 'Template',will wall off the lower pressure in front of it from the wake behind it.
A Porsche 914,Karmann Ghia,Lotus Europa,etc.,all have downward collision of air striking the body behind the backlight.The Lotus shelf extends far enough back to hit the 'Template'and will not allow the wake to migrate forward to the backlight.This is not the case for the 914 or Ghia.
If you watch the Porsche at El Mirage Dry Lake you can clearly see the dirt of the wake come right up behind the backlight and enter the engine inlet grille.Nasty!
Minimizing the circulation is something the spoiler(s) can help with,but even if pushed to the 'Template' you suffer some loss.

[aerohead]

Originally Posted by brucepick

Aerohead -
All other things being equal - which they never are of course,
would a spoiler angled upward 30º generate more downforce than a level one ending at the same point? And would that downforce carry a drag penalty?

Measured angles:

I measured some of the angles on the '96-00 Civic coupe using a print of the drawing that's been posted in these threads, and an old fashioned protractor. Also used a somewhat sketchy straightedge to establish the base lines, so absolute precision is not guaranteed!

The rear glass angle is about 20-22º downward, vs. horizontal. So, its local angle vs. the immediate prior roof is going to be less than 22º because the roof has already begun to curve downward at that point. This may explain the pretty good attached flow that I believe 98CaliformiaCivic found on the rear glass and trunk lid.
- - - tuft video
- - - damp tufts still #1
- - - damp tufts still #2

I measured the angle from roof to trunk lid edge, based from a point on roof slightly rear of the small side window's forward edge line. This is not the roof peak but is (to my eye) where it departs visibly from the template curve. I got 14º.

I measured from top edge of rear glass to trunk lid edge. I found about 16º there.
---------------------------------
I've seen recommendations for rear down angles of 14º from roof edge to trunk rear edge, and aerohead's recent suggestion to keep local angles at less than 22º. Based on those, it looks like the Civic coupe's stock back end is already in pretty good shape. However the trunk edge isn't anywhere near the template line. A "false trunk lid" to meet the template curve would be some 7"-9" higher than the original trunk, even if the false trunk lid is extended back a few inches to be even with the rear bumper. If not extended rearward, the false lid would need to be higher if it is to meet the template.

Aerohead, what do you think? Would there be gain in a false trunk lid if its sides are formed by extending the C-pillars rearward on their own existing angle? Or have the designers already worked the angles to the point where they've minimized the wake cross section - and a raised lid would only increase that cross section?

brucepick,there's a gray area with respect to your question.I have no data on direct comparisons between 'flat' spoilers and 'kicker' type spoilers ending at the same point.I don't like to guess.
Any spoiler,which terminated
'before' the 'Template'would permit comingling of the low pressure separated flow and general wake.
Spoilers terminating at the 'Template' would prevent this comingling,and wake pressure would be less affected.
The 'kicker' type spoiler would envelope a larger bird bath of vorticity than the flat spoiler and should have the higher drag.
Since the 'kicker' type has vertical architecture that the flat doesn't,it may
embody a vertical force vector impossible for the flat spoiler due to the lower base pressure acting upon it due to the higher vorticity in front of it.
Local tangent angles should be a function of the distance behind the point of max roof camber as per the 'Template'.This is why I created the 'Template',to get people away from talking about 'angles'.
Angles must be mentioned in the context of where they occur along the body structure.
Ideal 'angles' can be 'read' off the 'Template' for any position on the body.
If angles are 'hypo-'Template' the car will suffer increased drag due to separation.
If angles are 'hyper-'Template' the car will suffer increased drag due to increased skin-friction.
If you'll look at Mair's boat-tail graphic in the Phil Knox's Aerodynamic Photos Albums you'll see that even though 22-degrees will support attached boundary layer,it cannot be used until around one car height distance downstream of the point of max. roof camber.

[aerohead]

Originally Posted by Pendragon

aerohead,

I doubt that the airflow which would be affected by the spoiler you describe is laminar in flow. In fact, it should be rather turbulent. Isn't the spoiler simply moving the point of complete separation aft so that the area of the notch back is more stable, even if it is a "dead" area?

If one were to place a spoiler at the top of the rear window it should extend the separation some distance and then a second (lower) spoiler could extend the complete separation even further.

Pendragon,if the spoiler reaches up to the 'Template',the outer flow region will be laminar as seen in many wind tunnel smoke trace photographs.
The spoiler can help 'fix' a mediocre design by capturing a vortex in front of and ahead of it.The outer flow will skip over this region as if it were a solid structure.
You still must pay for the entropy of the circulation.

[Big Dave]

This is what I was driving at with my "Another Pickup Aerolid" thread.

After all, what is a pickup but a big notchback?

[Pendragon]

Originally Posted by aerohead

Pendragon,if the spoiler reaches up to the 'Template',the outer flow region will be laminar as seen in many wind tunnel smoke trace photographs.
The spoiler can help 'fix' a mediocre design by capturing a vortex in front of and ahead of it.The outer flow will skip over this region as if it were a solid structure.
You still must pay for the entropy of the circulation.

I do not want to get too far afield in the minutia of laminar flow, but true laminar flow is "attached" to the surface over which the fluid (air) is flowing. Even a well designed "laminar flow" wing experiences flow separation sooner than many people realize and the drag associated with the disturbed airflow. For example, there was an experimental aircraft which sucked this layer of disturbed airflow into the engine through a great many small holes. The experiment was successful in demonstrating the drag reduction possible, but the means of doing so was, and remains, impracticable. Certain production F-4s used engine bleed air to maintain lift in certain flight regimes. The drag reduction achieved by the use of flush rivets usefully improved aircraft performance.

Cars, on the other hand operate in a very disturbed airflow. That said, worthwhile improvements (reduction in drag) can be made despite the airflow not being truly laminar. We are seeing some changes in mass produced vehicles. Some of the "low hanging fruit" are more raked windshields and external mirrors that are designed to smooth airflow them.

You may recall that the Shelby Daytona Coupe had the point of maximum height of the roof line positioned well aft of the driver's seat for the specific purpose of delaying flow separation just as a laminar flow wing does.



Anyway, I have probably strayed

[some_other_dave]

Originally Posted by aerohead

If you watch the Porsche [914] at El Mirage Dry Lake you can clearly see the dirt of the wake come right up behind the backlight and enter the engine inlet grille.Nasty!

Nasty, but necessary. After all, the induction air and the cooling air all come from that engine bay. I believe the recirculation was deliberately used in the 914's case.

-soD

[aerohead]

Originally Posted by Pendragon

I do not want to get too far afield in the minutia of laminar flow, but true laminar flow is "attached" to the surface over which the fluid (air) is flowing. Even a well designed "laminar flow" wing experiences flow separation sooner than many people realize and the drag associated with the disturbed airflow. For example, there was an experimental aircraft which sucked this layer of disturbed airflow into the engine through a great many small holes. The experiment was successful in demonstrating the drag reduction possible, but the means of doing so was, and remains, impracticable. Certain production F-4s used engine bleed air to maintain lift in certain flight regimes. The drag reduction achieved by the use of flush rivets usefully improved aircraft performance.

Cars, on the other hand operate in a very disturbed airflow. That said, worthwhile improvements (reduction in drag) can be made despite the airflow not being truly laminar. We are seeing some changes in mass produced vehicles. Some of the "low hanging fruit" are more raked windshields and external mirrors that are designed to smooth airflow them.

You may recall that the Shelby Daytona Coupe had the point of maximum height of the roof line positioned well aft of the driver's seat for the specific purpose of delaying flow separation just as a laminar flow wing does.



Anyway, I have probably strayed

Straying is okay.
And I agree with all the boundary layer considerations you mention.
Aircraft are presumed to have fully attached flow and are primarily dominated by surface friction drag,hence the excitement over 'laminar' wings.
Motor vehicle drag is ruled by separation-induced pressure drag,hence the interest in streamlining the rear,moving the separation point back as far as practical/possible.
I was trying to emphasize what was happening way out away from the body in the flow region un-affected by the separated vortical flow and turbulence.
In road vehicle aerodynamics this region is referred to as laminar and is independent of the boundary layer conditions.
In the thread 'Flow-Images,' photos of smoke traces over vehicles taken in wind tunnels which depict undisturbed curvilinear paths over the vehicle are said to be depicting
'laminar' flow.

[Pendragon]

Originally Posted by aerohead

Straying is okay.
And I agree with all the boundary layer considerations you mention.
Aircraft are presumed to have fully attached flow and are primarily dominated by surface friction drag,hence the excitement over 'laminar' wings.
Motor vehicle drag is ruled by separation-induced pressure drag,hence the interest in streamlining the rear,moving the separation point back as far as practical/possible.
I was trying to emphasize what was happening way out away from the body in the flow region un-affected by the separated vortical flow and turbulence.
In road vehicle aerodynamics this region is referred to as laminar and is independent of the boundary layer conditions.
In the thread 'Flow-Images,' photos of smoke traces over vehicles taken in wind tunnels which depict undisturbed curvilinear paths over the vehicle are said to be depicting
'laminar' flow.

I am pretty much in agreement, but I do believe that placement of the max height of the roof line further aft than is commonplace (such as the Daytona Coupe) is something we should be seeing more of from the car designers.

I have my doubts about the wind tunnel smoke visualizations shown in commercials...I suspect that the positioning of the smoke wand is chosen to look good rather than show the problem areas. I suppose you can't blame them too much for turning the marketers loose.

As an aside, I have seen several things that the designers are "tweaking" that are probably beyond what is reasonable for all but the most committed modders. The shape of the headlight area of the nose of some cars has been shaped to modify airflow to try to reduce drag from the outside mirrors. The mirrors themselves have been seeing more attention to lower drag shapes as well. There are some other flow modifiers on the front fenders of some cars which attempt to reduce drag from the wheel well and wheel. I doubt that we will see the wind tunnel work for these items on commercials, but we should...it would demonstrate the attention to detail that helps make more efficient cars.

Cheers

P.S. When are we going to see flush mounted windshields and "hidden" windshield wipers?